1. Field of the Invention
The present invention relates to an image forming apparatus for forming an image with use of laser light, and an image forming method for use in the image forming apparatus.
2. Description of the Related Art
Heretofore, there has been generally used a laser scanner equipped with a rotary polygon mirror in an image forming apparatus such as a copier, a printer, or a fax. In the image forming apparatus, there is likelihood that the position of an image to be formed on a recording sheet may be displaced from where the image should be formed in a sub-scanning direction due to an attachment error regarding the position of a laser light source or the like. In such a case, the positional displacement in the sub-scanning direction can be corrected in the unit of scanning line by intentionally shifting an output of laser light by the distance corresponding to an appropriate number of scanning lines in the sub-scanning direction. This approach enables to suppress the positional displacement within ½ of the scanning line interval at maximum. The reason is as follows.
FIG. 5 is an illustration showing a scanning example according to the above approach. The solid line in FIG. 5 is a targeted scanning line i.e. a line representing the absolute position of scanning, and the dashed line in FIG. 5 represents an actual scanning line, with the arrow thereof showing the scanning direction. The actual scanning line 22 is displaced from the targeted scanning line 21 by about ½ of the scanning line interval, which corresponds to the resolution of the sub-scanning direction. In this case, if the contents i.e. image data to be outputted to the scanning line 22 is outputted to the position of a scanning line one line before the actual scanning line 22, a scanning line 23 is scanned. This means the displacement corresponding to about ½ of the scanning line interval is not canceled, and the approach fails to reduce the displacement. If the actual scanning line is displaced from the targeted scanning line by the amount more than ½ of the scanning line interval e.g. ¾ of the scanning line interval, the displacement can be reduced to the amount smaller than ½ of the scanning line interval i.e. ¼ of the scanning line interval by shifting the scanning line one line before the actual scanning line. This approach enables to suppress the displacement within ½ of the scanning line interval.
In the case where the rotation number of the rotary polygon mirror is increased without changing the rotating speed of a photosensitive member, each of the scanning line intervals can be decreased, with the result that the positional displacement is reduced to the amount smaller than ½ of the scanning line interval.
Also, Japanese Unexamined Patent Publication No. 09-52387 recites an image forming apparatus equipped with a galvano mirror in place of a rotary polygon mirror. The image forming apparatus performs image formation by reciprocal scanning with use of the galvano mirror, namely, by causing the galvano mirror to swing back and forth or reciprocate within a certain angular range at a fixed cycle.
In a tandem color image forming apparatus, a positional displacement may occur with at random displacement amounts in the respective colors. This leads to a displacement that relative scanning positions are displaced between the respective colors, which may cause a color displacement due to the displacement of the relative scanning positions between the respective colors.
In the image forming apparatus designed to increase the rotation number of the rotary polygon mirror, a noise may be increased by increasing the rotation number, or the useful life of a motor may be shortened, or use of a costly motor may be needed. Also, if the rotation number of the rotary polygon mirror is increased, a clock for outputting image data may be increased, which may lead to unduly increase of radiation.
Although the image forming apparatus recited in the above publication executes image formation by reciprocal scanning, the publication is silent about a method for correcting the positional displacement or the color displacement.